The present invention relates to a hot workable ferritic stainless steel alloy resistant to thermal cyclic stress and oxidation at elevated temperatures and having improved mechanical properties for use as substrate in exhaust gas purification applications, such as catalytic converters and heating applications.
In the description of the background of the present invention that follows reference is made to certain structures and methods, however, such references should not necessarily be construed as an admission that these structures and methods qualify as prior art under the applicable statutory provisions. Applicants reserve the right to demonstrate that any of the referenced subject matter does not constitute prior art with regard to the present invention.
Thin foils of ferritic Fexe2x80x94Crxe2x80x94Al alloys are today used as carrier materials for catalytic converters in the purification of exhaust gases from internal combustion engines. The biggest advantage of such an alloy lies in the formation of a thin, adherent aluminium oxide film on the surface. The surface oxide film protects the metal from rapid oxidation at the temperature at which the catalytic converter is used, and at cyclic thermal stress which are the usual working conditions in this application. In order to form such a protective oxide a minimum content of 4.5% aluminium in the alloy is assumed to be necessary.
The protective properties of this aluminium oxide are known to be improved, especially with respect to thermal cycling, if the alloy contains small amounts of one or more or the so called reactive elements (RE), such as Mg, Ca, Zr, Hf or rare earth elements (REM), such as Sc, Y or one of the lanthanide elements.
Such alloys can be produced conventionally by melting, refining, casting, billet rolling or forging followed by hot and cold rolling to produce thin strips with a final thickness of less than 150 xcexcm. The alloys can also be produced by using a pre-rolled strip with a lower aluminium content than the desired catalytic converter carrier material and subsequently depositing a layer of an aluminium rich alloy on the surface of this material. The deposition can be made in many different ways, e.g. by dipping the strip in a molten Al alloy, or by roll bonding (cladding) an Al alloy on top of a ferritic steel. A suitable method for coating by means of PVD-technology is described in U.S. Pat. No. 6,197,132, which is hereby incorporated by reference. This process can be used in order to deposit the Al rich alloy. In all these examples, the thickness of the strip with the deposit may be the final thickness, or the strip may be rolled down to a smaller thickness after the deposition has been performed. The composite of ferritic alloy and aluminium alloy may be heat treated to provide a homogeneous alloy, or an alloy with an increasing aluminium concentration towards the surface.
The mechanical properties of ferritic Fexe2x80x94Crxe2x80x94Al alloys, especially with increasing content of Al, are known to be poor at high temperature. Several ways of improving these properties are known, such as the production of fine dispersions of oxide or nitride phases by powder metallurgical processes. These processes involve expensive operations during production and are hence not suitable for the manufacturing of alloys that are to be produced in large quantities.
Another way of increasing the high temperature strength is by precipitating minute particles of nickel aluminides. This has been described for a number of Fexe2x80x94Nixe2x80x94Crxe2x80x94Al alloys with a basically ferritic structure or a mixed austenitic/ferritic structure.
However, such alloys are in present-day situations only known to be provided in hot rolled or cast conditions. If the alloys are to be used in catalytic converters, it must be possible to form the alloys by cold rolling down to a final thickness of less than approximately 100 xcexcm.
It is known to be possible to produce Fexe2x80x94Nixe2x80x94Crxe2x80x94Al alloys with a basically austenitic structure. The nickel content of such alloys should exceed 30 weight-%, which makes the raw material very expensive. The oxidation properties of such alloys are in general poorer than those of ferritic alloys.
It is known that the addition of approximately 10 weight-% nickel to a Fexe2x80x94Crxe2x80x94Al alloy furthermore improves the resistance to thermal shock, a phenomenon that is known to cause a reduction in the useful lifetime in the application of catalytic converters.
The future trend for metallic catalytic converter steel foils goes towards reduced thicknesses. This leads to several problems. One problem is the need of better oxidation properties because of less available aluminium per surface area unit. Another problem is that a material with increased high temperature fatigue strength will be needed.
It is therefore an object of the present invention to provide an alloy with increased oxidation resistance, resistance to cyclic thermal stress and improved mechanical properties such as increased high temperature fatigue strength for use as substrate in exhaust gas purifying applications, such as catalytic converters in combustion engines.
Another object of the present invention is to provide improved production process routes for a ferritic Fexe2x80x94Nixe2x80x94Crxe2x80x94Al alloy.
A further object of the present invention is to provide an alloy for use in heating applications.
According to one aspect, the present invention is directed to a ferritic stainless steel alloy resistant to thermal cyclic stress and oxidation at elevated temperatures having a composition comprising (in weight-%):
and
balance Fe and normally occurring steelmaking impurities and additions.
According to another aspect, the present invention provides a foil formed from a ferritic stainless steel alloy as set forth above the foil having a thickness of less than 150 xcexcm.
According to another aspect, the present invention provides a heat treating furnace comprising a component formed from the alloy as described above.
According to yet another aspect, the present invention provides a catalytic converter comprising a component formed from the alloy as described above.
According to a further aspect, the present invention provides a catalytic converter comprising a component formed from the foil as described above.
According to an additional aspect, the present invention provides a method of producing a foil of an alloy having a composition as set forth above, the method comprising coating a substrate material with pure aluminium and/or an aluminium-base alloy by dipping, cladding or PVD.